As is known in the art, a digital color image can be stored or expressed as three-dimensional values of picture elements ("pixels") that in combination form the image. Each pixel value corresponds to a location in a color space which represents a particular color. Often used color spaces include a so-called red-green-blue (RGB) color space and a so-called cyan-magenta-yellow (CMY) color space. In the RGB color space, each pixel is provided from combinations of red, green and blue pixel components or pixel vectors having predetermined magnitudes. The red, green and blue pixel components are thus combined to provide the pixel having a particular color. Similarly in the CMY color space each pixel is provided from combinations of cyan, magenta and yellow pixel components having predetermined magnitudes which are combined to provide the pixel having a particular color.
Image processing systems typically represent each pixel component with a predetermined number of bits. For example, each pixel component may be represented by eight bits. In this case each pixel component may be assigned one of 2.sup.8 or 256 possible values. In such a case, each pixel component can have a value between 0 and 255 for example.
Ink jet printers typically print by disposing a plurality of like-sized ink dots on a recording media such as paper. The ink dots are placed on the recording media in locations corresponding to pixel locations in the image. The printer is binary in the sense that the ink dot is either applied or not applied to the recording medium at any given pixel location. Intensity levels and colors in an original image which cannot be directly produced by binary printers are simulated in printed images using a technique generally referred to as half-toning.
In a halftoning technique, colors and intensities different from that which a printer can directly provide are produced by applying printer-ink dots in different percentages of the pixels. In ordered dither-type half-toning, threshold values of a dither matrix are associated with respective display-medium pixel locations and the printer deposits ink on a recording medium at image locations at which the pixel component value equals or exceeds the associated threshold value of the dither matrix. Thus, if the dither matrix includes threshold values which are more or less evenly distributed throughout a range (e.g. 0-255), a pixel component having a value in the middle of the range (e.g. a value of 128) to results in a dot of ink having a color associated with that pixel component being deposited at approximately one-half of the pixel locations. Similarly, a pixel component having a value near or at the top of the range of threshold values (e.g. a value of 255) typically results in a dot of ink having a color associated with that pixel component being deposited in all pixel locations at that region of the image. When dots are deposited in all locations of an image, ink is said to be deposited with a 100% ink duty. A 100% Ink duty represents the maximum per unit area quantity of ink that can be deposited on a recording media.
As is also known, in a clustered-dot dithering method, dots are clustered in patterns. Thus, a cluster dot typically includes multiple printer ink dots. Cluster dot size is increased by printing a first ink dot at an initial point defined by a dither matrix and printing subsequent ink dots in a spiral pattern emanating from the initial point.
One problem which arises when printing using the conventional clustered-dot dither approach is that since the ink-dots are closely spaced, portions of adjacent printer dots overlap. Furthermore, close inspection of such printer ink dots reveals that when horizontally or vertically adjacent dots are printed at maximum ink duty limits, overlapping regions of the dots correspond to local regions within the cluster dot where ink bleeding occurs. Ink bleeding refers to deposition of a quantity of ink on the recording media which is greater than the amount of ink the recording media can absorb and the resultant flow of ink away from the area in which the excessive amount of ink was deposited.
For example, if it is desired to print ink dots of cyan, magenta and yellow color to thus produce a black dot, a 100% ink duty cannot be used for each pixel component color because the total ink duty, and thus the amount of ink disposed on the recording medium, will typically be greater than that which the recording medium can reasonably absorb. This results in ink running or bleeding on the recording medium.
In addition to the above-mentioned ink duty limit problem, the spiral pattern in which the ink dots are deposited on the recording medium to form the cluster dots also results in consecutively printed ink dots having portions which overlap. For example, in conventional cluster dot dithering, when four dots are printed they are arranged in a square pattern and overlap at a point which is located at the center of the four dots. In a case where only four dots out of a possibility of sixteen dots are printed, for example, no violation of a total ink duty limit of the image region occurs because only four dots out of sixteen dots have been printed. The image, however, can appear relatively noisy to a viewer due to bleeding. This noisy image appearance is due, at least in part to, the excessive amounts of ink and the ink dot overlap problem.
In ink jet printers printing with conventional clustered-dot dithering techniques, the overlapping problem is amplified for at least two reasons. First, the printer ink dots are often larger than a corresponding space which the ink dots are ideally suited to fill. Second, dot overlap occurs because ink dots disposed on the recording media tend to spread or expand in size upon impact with the recording media. The amount by which the dots expand depends upon a variety of factors including paper quality (e.g. glossy paper vs. non glossy paper), ink composition, etc . . . Dot spreading results in further overlap of adjacent dots and is further exacerbated by depositing excessive amounts of ink on the recording media.
Moreover, imperfect horizontal printer alignment or registration also results in ink dot overlap. The overlapped regions of the dots appear in the image as dark or light bands typically referred to as banding artifacts. Thus, an image printed using a clustered dot dithering technique is susceptible to banding artifacts caused by overlap of proximate printer dots and due to a variety of factors including printer registration problems. The bleeding and banding artifacts are especially noticeable to a human viewing the image when the image includes regions having ink dots printed with a relatively low ink duty (i.e. an ink duty in the range of 0% to about 25% ink duty).
Printer misregistration is typically caused by mechanical tolerances and inaccuracies in the electromechanical design and/or manufacture of the printer. One particular cause of misregistration are the inaccuracies of the printer gearbox and drive mechanisms, including pulleys, belts, worn gears and lead screws. The variations in the gear operation produce different frequencies of banding.
To avoid or minimize the occurrence of the aforementioned image artifacts, the printer must accurately position dots on the recording medium to produce regularly spaced cluster dots. However, while the misregistration can be improved by improving mechanical tolerances in both printer components and assembly of the printer components, this is typically costly both in the design and the manufacturing processes. Thus, due to the inability of the printers to provide perfect registration in horizontal and vertical directions, the printer cannot position the printer dots in the cluster in precisely the correct location which results in the printed image having artifacts.
When portions of adjacent printer dots overlap do to the printer mis-registration, the printer dots are said to be "torn apart" or "sheared." When cluster dots are sheared, changes in the intensity of the image result, due at least in part, to the overlapping of some cluster dots and the large spacing between other cluster dots. This is especially noticeable in those regions of a printed image in which ink duty limits of about 25% or less are used. Thus, shearing results in the printed image having banding artifacts.
Thus, even if the recording media could absorb the ink disposed thereon by the printer, due to the printer registration problem, the banding artifacts would be particularly noticeable if each ink were allowed to be deposited with a 100% ink duty. Conversely, even if a delivery mechanism (e.g. a print head) could accurately stop at every 1/720 inch position, for example, the printer ink dot would still spread over a relatively large area of the recording media causing adjacent dots to overlap resulting in the imaging having the aforementioned banding artifacts and bleeding characteristics. Thus, the ink bleeding and printer registration problems are intertwined.
It would therefore be desirable to provide a technique for reducing the banding artifacts resulting from a printer generating a halftone image using a clustered-dot dithering technique. It would also be desirable to reduce the ink bleeding in images.